1. Field of the Disclosure
The present invention relates generally to analog signals, and more specifically, the invention relates to processing analog signals.
2. Background
Conventional comparators make a comparison between an instantaneous value of an analog signal and a reference. A time-differential analog comparator compares values of an analog signal gathered over a present time interval with values of the same analog signal gathered over a past time interval. It is useful in some applications of electronic circuits to know whether or not a slowly changing quantity has changed significantly over a relatively long period of time for the circuit.
Electronic circuits typically use components that are optimized for operation at high speeds, responding to events that occur in very short times. Those circuits are often used in applications to control slowly varying processes that take much longer times to respond. For example, the components of the miniature circuits that regulate large mechanical systems and to control industrial processes typically are scaled to operate with events that occur over periods of microseconds. Those circuits often must respond to events that occur over periods of several seconds, minutes, or hours. It is not practical to increase the size of the circuit to match the slow response of the application.
A common technique that allows small, fast circuits to work with a slow, analog process converts the analog information into digital form and then stores the digital information in a memory. The fast circuit either waits in a suspended state or performs other tasks until sufficient time passes for a change to be likely in the analog information. Then the present analog information is converted to digital form and compared to the past analog information that has been stored in memory. This technique is natural in larger systems that use analog to digital converters, digital memory, and digital processors in their control circuits. The technique is usually not practical for simple, low-cost systems that do not otherwise require analog-to-digital conversion and digital memory in their control circuits.
An alternative technique that stores the analog information as an analog value with a sample and hold circuit for use with a conventional comparator is also not practical when long intervals of time are involved, particularly when it is desired to package the control function in an integrated circuit, as the use of such alternative techniques generally involves large capacitors and very small currents that approach the values of leakage currents.